1. Field of the Invention
The field of the invention is that of methods for reducing acoustic noise in a speech signal.
2. Description of the Prior Art
When sound is recorded in or transmitted from a noisy environment it is advantageous to eliminate the background noise so that this is not recorded or transmitted by the means for recording or transmitting the sound signal. The latter situation is encountered in the field of mobile radiotelephones where it is desirable to avoid transmitting the background noise, for example the engine noise of a vehicle in which a radiotelephone is used, to the recipient of the speech signal.
The article "Acoustic noise analysis and speech enhancement techniques for mobile radio applications" by DAL DEGAN and PRATI, Elsevier Science Publishers B. V., Signal Processing 15, 1988, pages 43 through 56 describes and compares various techniques for processing noise in a speech signal from an automobile vehicle.
According to this article, there are known signal processing methods which estimate the background noise spectrum and subtract this noise spectrum from the spectrum of the measured signal produced by a microphone. This method based on the principle of noise cancellation by spectral subtraction is also described in the article "Suppression of acoustic noise in speech using spectral subtraction" by S. F. BOLL, IEEE Trans. ASSP. Vol. ASSP-27, 1979, pages 113 through 120.
However, the main drawback of this method is that the noise spectrum must be updated frequently to allow for changes in the background noise and this can only be done when the user is not speaking, i.e. during periods of silence. Thus in an environment in which the background noise varies frequently and significantly, and in particular in an automobile vehicle, numerous periods of silence are required for frequent updating of the background noise spectrum. Periods of silence which are sufficiently long for updating the noise spectrum are not always available and if the gaps between periods of silence are too long the noise spectrum is degraded and can no longer be used to compensate for noises of short duration. This degrades the quality of the speech signals transmitted.
One object of the present invention is to alleviate these drawbacks.
To be more precise, one object of the invention is to provide a sound signal processing method enabling background noise to be attenuated significantly and the quality of a transmitted speech signal to be improved accordingly, the noise being attenuated using a noise spectrum which is updated without requiring periods of silence by the speaker.